Technical Field
The present invention relates to a photoacoustic imaging method, namely, a method for imaging a subject, such as a living tissue, based on photoacoustic waves emitted from the subject when it is exposed to light outputted toward the subject.
The present invention also relates to a device that carries out the photoacoustic imaging method.
Background Art
As disclosed in Japanese Unexamined Patent Publication Nos. 2005-021380 and 2011-217767 (hereinafter, Patent Documents 1 and 2, respectively) and X. Wang et al., “A High-Speed Photoacoustic Tomography System based on a Commercial Ultrasound and a Custom Transducer Array”, Proc. of SPIE, Vol. 7564, pp. 756424-1-756424-9, 2010 (hereinafter, Non-Patent Document 1), for example, a photoacoustic imaging device for imaging the interior of a living body using the photoacoustic effect is conventionally known. With the photoacoustic imaging device, pulsed light, such as pulsed laser light, is outputted toward the living body. In the interior of the living body exposed to the pulsed light, a living tissue absorbs the energy of the pulsed light and the volume of the living tissue expands due to heat, and acoustic waves (photoacoustic waves) are emitted. By detecting the photoacoustic waves with a detection means, such as an ultrasound probe, the interior of the living body can be visualized based on the obtained electric signals (photoacoustic signals).
The photoacoustic imaging device constructs an image based only on photoacoustic waves emitted from specific absorptive substances, and is therefore suitable for imaging a specific tissue, such as blood vessels, in a living body.
With the photoacoustic imaging device, a tissue at a deeper position than the surface of the subject, such as blood vessels of a living body, can be imaged, as described above. To this end, it is necessary to set a relatively high intensity of the pulsed light so that the pulsed light can reach deep positions. In this case, however, near-surface parts of the subject (such as the epidermis and the body hair) exposed to the pulsed light emit photoacoustic waves, and a photoacoustic image of the near-surface parts may be generated and displayed. The thus displayed near-surface parts may hinder observation of an intended part to be observed, such as blood vessels, or may even hide the part to be observed.
Patent Document 1 also discloses a method for solving this problem. This method involves applying a Fourier transform in the spatial direction to photoacoustic wave detection signals obtained with a plurality of detection elements to cut off spatial low-frequency components, applying an inverse Fourier transform to the signals, and generating and displaying a photoacoustic image using the thus converted image signals.